Engineering of a Date Library

It Started as a Helper Func.

The JavaScript Date API? Clunky, confusing, and lowkey cursed. Like, who thought returning 0-11 for getMonth() was a vibe? Or getYear() spitting out 123 for 2023? Absolute chaos. Out of that pain came a tiny helper function for formatting dates, adding/subtracting days, and handling basic timezones. After copy-pasting it into way too many side projects, I realized: dates are just math with extra drama. That’s when waktos was born — a 5kB library designed to end developer suffering in JavaScript date-land.

Everything is Timestamp

At the core, dates boil down to numbers. Unix timestamp = seconds since January 1, 1970. If you’re in UTC, it’s just adding and subtracting integers. Pure math, no nonsense.

// Short version
return createInstance(
  this._timestamp + value * multiplier,
  this._locale,
  this._timezone,
);

Why timestamp-first?

• Lean memory usage (~32 bytes per instance)
• One single source of truth (no sync headaches)
• UTC ops = fast math mode

Quick benchmarks show: ~42% faster instance creation, ~67% less memory footprint.

Map Cache Hack

Intl.DateTimeFormat is heavy (seriously) — like 10–50ms per call heavy. Solution? cache! At first I tried objects, but implementing LRU (Least Recently Used) got messy. Then I found out Map in JS has a secret move: delete + set pushes items to the back, making a dead-simple LRU.

#bump(key, value) {
  this.#cache.delete(key);
  this.#cache.set(key, value);
}

Result? ~90% cache hit rate after warmup. Cold start penalty exists, but subsequent operations are lightning fast. Smooth, fr.

Timezone Hell

DST (Daylight Saving Time) is the global prank humanity decided to play on programmers. Offsets jump an hour out of nowhere. Waktos tackles this with an iterative refinement algorithm:

1. Calculate initial UTC based on estimated offset:

   let estimatedUtc = local - offsetEstimate * MINUTE;

2. Get actual offset for estimated UTC:

   const initialOffset = calcTimezoneOffset(estimatedUtc, tz);

3. Adjust calculation based on difference:

   estimatedUtc -= (initialOffset - offsetEstimate) * MINUTE;

4. Test refined calculation:

   const refinedOffset = calcTimezoneOffset(estimatedUtc, tz);

5. Choose smaller offset to avoid double-counting:

   const finalOffset = Math.min(initialOffset, refinedOffset);

Better to be roughly right than precisely wrong. Pragmatic approximation is more valuable than perfect accuracy that's fragile.

Two-Pass Formatting

Date formatting has two jobs: keep literal text safe, swap tokens with values. Example: [Born on] YYYY-MM-DD. Solution? Two passes. First capture literals, then replace tokens.

const parsePattern = (pattern, locale) => {
  // Pass 1: literals
  // Pass 2: tokens
};

And lazy evaluation with function closures. Token values are computed only when needed, with locale-aware number formatting.

Human Psychology in Code

Tiny tweaks, big vibes:

• Nobody says "1 week ago" — it’s always "last week".
• Anything under 5 seconds feels like "now".
• RTL languages (e.g. Arabic) get proper Unicode bidi love.

These details make the library feel natural to humans, not just machines.

Extensibility

Still tiny, but pluggable:

waktos.plugin = (...plugins) => {
  /* ... */
};

With TypeScript module augmentation, extensions stay type-safe and clean.

Lessons

• Trade-off: Timestamps make component access a bit verbose, but the perf wins are worth it.
• Error handling: Better roughly right than precisely wrong.
• Bundle discipline: Every feature got weighed against the 5kB cap.
• DX first: Months are 1-based, because dev brains > raw efficiency.

Reflections

The invisible stuff — makes or breaks the experience. When it works, no one notices. When it doesn’t, everyone does. Waktos taught me that performance is about trade-offs, defensive programming is insurance, and small details punch way above their weight.

End Result?

A lightweight, fast, and delightfully human JavaScript date library:

• Get started?
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